A key step in the production of magnesium metal by the electrolysis of magnesium chloride is the preparation of the anhydrous magnesium chloride feed. Several processes are used commercially to produce anhydrous magnesium chloride. The oldest is the process of IG Farben, wherein briquettes of MgO and coke are reacting with chlorine in an electrically heated vertical shaft furnace to produce molten magnesium chloride at about 800.degree. C. Major drawbacks of this process include its low production activity (&lt;30 tpd of molten MgCl.sub.2 per furnace), periodic shutdowns required to remove unreacted residues from the bottom, high chlorine requirement, and the presence of chlorinated hydrocarbons in the exhaust gas.
U.S. Pat. No. 4,269,816 proposes a shaft furnace chlorination process producing molten anhydrous MgCl.sub.2 directly from lump magnesite ore using CO as the reductant. This process has the advantage of eliminating the magnesite to MgO calcination and MgO/coke briquetting steps, but it requires a very pure magnesite teed to make magnesium metal of good quality, and it still has not resolved the remaining drawbacks of the IG Farben chlorinator, that is, low productivity and chlorinated hydrocarbon emissions.
Norsk Hydro has developed a process for producing anhydrous MgCl.sub.2 prills from concentrated MgCl.sub.2 brines. This process is described in U.S. Pat. No. 3,742,100 and consists in a) evaporating MgCl.sub.2 brine to a concentration of up to 55% MgCl.sub.2 ; b) prilling the concentrated MgCl.sub.2 brine to form prills of MgCl.sub.2.4-6 H.sub.2 O of suitable size for fluidized bed processing; c) fluidized bed dehydration with air at 200.degree. C. to produce MgCl.sub.2.2H.sub.2 O powder; d) three-stage fluidized bed dehydration with anhydrous HCl gas at about 300.degree. C. to give anhydrous MgCl.sub.2 powder containing less than 0.2% each by weight of MgO and H.sub.2 O. This process is operating commercially but requires recirculation of very large mounts of HCl gas, for example up to 50 times the stoichiometric requirement for dehydration, and, hence, is very complex and capital intensive.
U.S. Pat. No. 3,953,574 discloses a process that produces molten MgCl.sub.2 by reacting spray-dried MgCl.sub.2 powder containing about 5% by weight each of MgO and H.sub.2 O, with a solid carbonaceous reductant and chlorine gas at a temperature of 800.degree. C. The process is carried out in two in-series rectangular furnaces, heated electrically via graphite electrodes installed in the furnace walls. Spray-dried MgCl.sub.2 is fed with a solid carbon reductant into the first furnace and chlorine gas is bubbled through both furnaces using graphite lances to react MgO and H.sub.2 O in the feed to MgCl.sub.2 and HCl respectively. The final MgCl.sub.2 melt contains less than 0.5% MgO. However, it has been found that in order to obtain sufficiently high chlorine utilization efficiencies, it is necessary to provide ferrous chloride to the melt either by adding an iron metal or oxide to the chlorination furnace or preferably by adding ferrous chloride solutions to the MgCl.sub.2 brine before spray drying. Without such iron additions, chlorine efficiencies of less than 40% were achieved which would be too low for a commercial process. However, the use of iron results in several drawbacks, namely, the residual iron level of 0.5% in the product MgCl.sub.2 melt decomposes in the electrolytic cell to iron metal that accumulates as sludge and causes losses in cell operating efficiency. Also, part of the iron added volatilizes causing stack emission problems. The residual iron level in the MgCl.sub.2 product from the above process is too high for use in modem, sealed electrolysis cells, and therefore, a bipolar pre-electrolysis operation had to be developed to bring the iron level down to less than 0.1% in order to use its MgCl.sub.2 as a feed to such cells (U.S. Pat. No. 4,510,029).
In U.S. Pat. No. 4,981,674, a process for preparing anhydrous MgCl.sub.2 is described. The d process comprises the steps of feeding spray-dried MgCl.sub.2 powder, magnesite or magnesia powder into molten MgCl.sub.2 in a furnace at a temperature of 750.degree.-850.degree. C.; and adding gaseous reactants like chlorine and carbon monoxide through a gas disperser located within the molten MgCl.sub.2 bath to produce fine gas bubbles that will react with the MgO in the bath and reduce its level down to less than 0.1%.
Australian Patent 120,535 teaches feeding hydrated magnesium chloride into a separate chamber of molten electrolyte, containing 10-55% MgCl.sub.2 at a temperature in excess of the normal electrolytic cell temperature of 725.degree.-750.degree. C., preferably 800.degree.-850.degree. C., to decompose magnesium hydroxy chloride and increase the MgCl.sub.2 concentration in the separate chamber to a level up to 50% MgCl.sub.2. Magnesium oxide formed during this process can be partially reacted by introducing a chloridizing agent, such as hydrochloric acid gas or carbon and chlorine into this chamber. Periodically, the MgCl.sub.2 -enriched electrolyte is transferred batchwise to the adjacent electrolysis chamber. Also, MgO-containing sludge formed in the chamber must be removed periodically by dredging. The process described in this patent operates at temperatures higher than 750.degree. C. At such high temperatures, most of the hydrated MgCl.sub.2 fed is hydrolyzed to MgO. If hydrogen chloride gas is used as the chloridizing agent, large quantities of gas are required at such high temperatures to reduce the MgO level in the electrolyte to levels sufficiently low for modem, sealed electrolysis cells. The quantity of dry HCl required is well in excess of the 2 moles of HCl per mole of MgCl.sub.2 in the hydrated MgCl.sub.2 feed available from the chlorine produced by the electrolysis of magnesium chloride. Hence, the HCl gas must be recycled through a complex, drying system, such as that described in U.S. Pat. No. 3,779,870; the latter system being very expensive to install and operate. This process is also not applicable to modem, sealed magnesium electrolysis cells as it does not reduce the MgO level in the MgCl.sub.2 -enriched electrolyte to a level sufficiently low, that is, typically less than or equal to 0.1% MgO on a 100% MgCl.sub.2 feed basis, to ensure economic cell operation. Also, removal of MgO containing sludge from electrolyte is an unpleasant and inefficient process.
Japanese Patent 32-9052 describes the feeding of hydrated magnesium chloride to a magnesium chloride-containing electrolyte (25% MgCl.sub.2) while introducing dry HCl gas at a temperature of 750.degree. C. Without dry HCl gas, 22% of the MgCl.sub.2 introduced reacted with the moisture in the feed to form MgO. In the various examples given, it is stated that MgO formation is almost completely prevented when injecting HCl at levels equivalent to and slightly higher than the 2 moles of HCl per mole of MgCl.sub.2 in the hydrated MgCl.sub.2 feed to be produced from the chlorine from the subsequent electrolysis of the MgCl.sub.2 formed. However, subsequent electrolysis of the MgCl.sub.2 -electrolytes gave graphite consumption levels in the range of 13-15 kg graphite/toe magnesium metal produced. This is 20 to 30 times greater than the maximum permissible graphite consumption in a modem, sealed magnesium electrolysis cell. Therefore, although the patent states that there is substantially no MgO in the electrolyte, the graphite consumption suggests otherwise, that is, a high MgO level. This is highly likely since MgO readily settles out as a sludge if the electrolyte is not agitated efficiently, as would be expected if HCl gas was only bubbled in, as shown in the FIG. 2 of the Japanese patent. The resulting MgO sludge at 750.degree. C. is still fluid, and it can therefore be easily resuspended by the circulating electrolyte. Thus, the process as described in this patent, is not capable of supplying in MgCl.sub.2 electrolyte containing loss than 0.1% MgO.
Modern magnesium electrolysis cells, such as the Norsk Hydro monopolar cell (U.S. Pat. No. 4,308,116) and the Alcan multipolar cell (U.S. Pat. No. 4,560,449), are referred to by those skilled in the an as "sealed" cells, since they are very tightly sealed to prevent the ingress of moist air. These so-called sealed cells are designed to operate for several years without stopping the cell operation. Accordingly, the graphite anodes cannot be changed and the sludge cannot be removed from the cell without closing the cell down. Since the rebuilding of sealed cells is very costly, it is imperative that the starting material fed to the cell, namely anhydrous magnesium chloride, contains very low levels of MgO, preferably less than 0.1% by weight. This is motivated by the fact that the MgO present in the feed or formed in the cell by the ingress of moist air will either react with the graphite anodes to consume graphite or form a magnesium-oxide containing sludge.
Commercial MgCl.sub.2 electrolysis cells operate typically with a MgCl.sub.2 level in the molten electrolyte in the range of 10-20% MgCl.sub.2 with the remainder of the electrolyte comprising a mixture of NaCl, CaCl.sub.2 and KCl in various proportions depending on the purity of the MgCl.sub.2 feed. A typical electrolyte composition is about 60% NaCl, 20% CaCl.sub.2, 0-5% KCl and 15-20% MgCl.sub.2.
Graphite consumption leads to an increase in the anode-to-cathode distance, resulting in an increased cell operating voltage required, thus causing an increase of power consumption per unit of magnesium produced. As a result, the cell must be shut down when either its power consumption per unit of magnesium becomes too high for continued economic operation, or the cell heat balance can no longer be maintained. Any sludge formed in a sealed cell sinks and eventually forms a concrete-like mass on the bottom of the cell If sludge formation is excessive, it will disrupt electrolyte flow in the cell sufficiently to close the cell.
It, therefore, becomes apparent that in view of the foregoing, hydrated magnesium chloride is not added to "sealed" cells because moisture reacts with either magnesium chloride or magnesium metal to form MgO, or reacts directly with the graphite anode to consume it. This is confirmed by an article from Dow Chemicals in Kirk-Othmer Encyclopedia, Volume 14, pages 570-615, wherein it is observed that the addition of hydrated magnesium chloride powder containing 1.5-2.0 moles H.sub.2 O per mole of MgCl.sub.2, directly to a specially designed electrolysis cell results in the generation of MgO that forms sludge that must be removed manually from the cell daily to prevent a permanent build-up and subsequent interference with cell operating efficiency. A part of the moisture added in the feed also reacts with the graphite anodes, leading to a very high graphite consumption of about 0.1 ton graphite per ton of magnesium produced. To avoid cell operating disruptions due to this high graphite consumption, the Dow electrolysis cell is designed with consumable graphite anodes that are periodically lowered into the cell to maintain the same anode to cathode distance. The high graphite anode consumption in the Dow cell is a major cost and it also means that the Dow cell cannot be designed with a small anode to cathode distance. A high power consumption of over 15,000 kWh/ton Mg metal is, therefore, required, compared to only about 10,000 kWh/ton Mg for modem sealed cells. Further, the off-gas from the Dow cell is a dilute chlorine gas (less than 30% Cl.sub.2) contaminated with high levels of H.sub.2 O, HCl, CO, CO.sub.2, H.sub.2 and N.sub.2 that cannot be used to recover chlorine gas for recycle or sale, if required. Modem sealed cells produce a concentrated chlorine gas stream, containing more than 95% Cl.sub.2.
Accordingly, there is a great need to develop a simple process for the production of magnesium metal from anhydrous MgCl.sub.2 in sealed cells that would reduce to the minimum the consumption of the graphite anode, and significantly diminish the production of sludge in the cell Such process would certainly be of great benefit if the currently used anhydrous magnesium chloride could be replaced with hydrated magnesium chloride as the starting material, the latter being significantly less difficult to produce. Previous attempts to feed hydrated MgCl.sub.2 into cell electrolyte as described in Australian patent 120,535 and Japanese patent 32-9052 use temperatures of 750.degree. C. or higher and do not produce MgCl.sub.2 -containing electrolyte with the very low levels of MgO needed and without eliminating undesirable sludge formation. Also, it is very important to reduce the HCl requirement to significantly less than 2 moles of HCl per mole of magnesium chloride produced from hydrated magnesium chloride that could be produced from the chlorine gas from the electrolysis cell, such that costly HCl gas drying and recirculating systems do not have to be used.